Paper and paperboard are often sized with various hydrophobic compounds including, for example, wax emulsions, ketene dimers, isocyanate derivatives, fatty acid complexes, fluorocarbons, certain styrene-maleic anhydride copolymers, and substituted cyclic dicarboxylic acid anhydrides such as alkenyl succinic anhydride (ASA). These compounds are referred to as sizes or sizing compounds and may be introduced during the actual paper making operation wherein the process is known as internal sizing. Sizing compounds may also be applied to the surface of the finished web or sheet in which case the process is known as external or surface sizing.
In the case of ASA size, in order to have effective sizing agents with ability for uniform size dispersion throughout the fiber slurry it must be emulsified into a fine particle size emulsion. Typically such emulsions are prepared with the aid of ionically modified starches, carboxymethyl cellulose, natural gums, gelatin, synthetic polymers, or polyvinyl alcohol, all of which act as colloidal stabilizers. In commercial practice, ASA is emulsified with these materials with or without surfactant. The desired particle size for a good quality emulsion is a majority of the particles having a particle size of less than 2 microns. Emulsions having an average particle size of greater than 2 microns negatively affects the retention mechanism and creates deposits of undesired hydrolysates throughout the papermaking process. This adds to the product quality problems and necessitates disassembly and cleaning of the papermaking equipment.
Most presently used emulsification systems are designed on high homogenizing shear and/or pressure, and high tolerance between the stationary and rotating blades, with a flow recirculation loop. In these systems, emulsion prior to attaining its workable property must pass through the turbine emulsification apparatus multiple times, generally in excess of nineteen passes. This high shear, heat generating process has a negative impact on the quality of the final product. It also adds to the cost and complexity of the equipment since extra controls and additional equipment/space is required for the process.
Therefore, it would be advantageous to prepare a paper size emulsion having an average particle size of less than 2 microns through one-pass, non-tolerance dependent dispersion system.